The significant polarization components of a medium produced by contact with an electric field are first order polarization (linear polarization), second order polarization (first nonlinear polarization), and third order polarization (second nonlinear polarization). On a molecular level this can be expressed by the equation: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 . . . ,
where P is the total induced polarization, E is the local electric field created by electromagnetic radiation, and .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. .beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. On a macromolecular level corresponding relationships can be expressed by the equation: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3 . . . ,
where P is the total induced polarization, E is the local electric field created by electromagnetic radiation, and .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order electric susceptibilities of the electromagnetic wave transmission medium. .chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear electric susceptibilities, respectively, of the transmission medium.
Second order nonlinear optical (NLO) effects are useful for a variety of optical devices, including electrooptic modulation and frequency conversion. The bulk second order susceptibility, .chi..sup.(2), represents a "figure of merit" for the performance of a material in an NLO device. To achieve on a macromolecular level second order polarization (.chi..sup.(2) E.sup.2) of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) electric susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). This typically corresponds to a value of the first hyperpolarizability, .beta., of greater than about 10.sup.-30 esu.
In attempts to gain increased .chi..sup.(2), researchers have turned to organic materials over traditional inorganic materials such as lithium niobate. A large number of organic chromophores have been synthesized that possess substantial second-order NLO properties. A significant difficulty encountered in finding suitable materials for second order polarization effects lies in the molecular requirements that must be satisfied to achieve usefully large values of .beta.. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, noncentrosymmetric. It has been observed experimentally and explained by theory that large .beta. values can result from large differences between ground and excited state dipole moments as well as large oscillator strengths, characteristics commonly displayed by charge-transfer molecules. A typical chemical structure that provides these effects has the combination of an electron donor moiety and an electron acceptor moiety linked by a conjugated .pi.-molecular orbital framework. The terms "electron donor moiety" and "electron acceptor moiety" are used herein, as those terms are typically used in the art, to refer to groups or atoms having negative and positive values, respectively, of the Hammett .sigma.para substituent constant (also referred to herein as "Hammett-sigma"). Unless otherwise indicated, Hammett-sigmas stated herein are from Lange's Handbook of Chemistry, 13th edition, ed. John A. Dean, McGraw-Hill Co., New York, page 3-135 to page 3-140. The term "molecular dipole" is commonly used to designate a molecule or moiety that can be represented by the general structure ##STR1## where D is an electron donor moiety, L is a linking moiety and A is an electron acceptor moiety. The linking moiety has a conjugated .pi. bonding system, which provides a pathway for charge transfer resonance between the electron donor moiety and the electron acceptor moiety. A molecular dipole can be a discrete molecule or a moiety which is a part of a larger molecule, such as a group pendent from a macromolecular backbone.
For a bulk material to exhibit nonzero .chi..sup.(2), the molecular dipoles must be arranged in a noncentrosymmetric fashion. One of the most common techniques for establishing this arrangement is electric field poling. For an initially amorphous or glassy system that is made noncentrosymmetric by electric field poling, the following expression holds: EQU .chi..sup.(2) .varies.N.beta.* L(E.mu./kT).
.beta. is the molecular first hyperpolarizability. N is the concentration of molecules which exhibit NLO activity, that is, the concentration of molecules, which each include at least one molecular dipole. L is a third order Langevin function that takes values between 0 and 1. E is the electric field applied during poling. .mu. is the ground state dipole moment. T is the absolute temperature. k is Boltzmann's constant. For typical poling voltages and dipole moments, the Langevin function is linear with respect to its arguments and the equation can be simplified to: EQU .chi..sup.(2) .varies.NE.mu..beta./kT.
Many of the quantities in this expression are subject to practical limitations. The concentration of NLO-active species cannot be made much greater than approximately 10.sup.22 molecules/cm.sup.3. The poling voltage cannot be higher than about 300 V/.mu.m without the probability of cataclysmic dielectric breakdown in the sample. Alignment is not retained after poling a material unless the temperature during poling is at or above the glass transition temperature (Tg) of the material, which in turn must be well above ambient temperature to prevent subsequent relaxation of the induced order upon cooling to ambient temperature and removal of the poling field. Therefore .mu. and .beta. represent the principal quantities subject to modification in efforts to develop improved NLO materials. On this basis the product .mu..beta. represents a figure of merit for an NLO material that is to be oriented by poling.
While there is no theoretical limit to the magnitude of .beta. for organic molecules, in practice it has been found that an increase in the nonlinearity, .beta., is accompanied by a red-shift in the absorbance band. Since the propagated light in NLO devices is usually visible or near-infrared, it is desirable that the NLO material be completely transparent at those wavelengths. Even a small absorbance causes undesirable attenuation and in some cases deleterious photochemical reactions or localized heating of the material. These effects are major shortcomings which can substantially decrease practical NLO performance.
A similar shortcoming has been found for increases in .mu., the dipole moment of the molecule to be poled. More polar NLO chromophores generally possess red-shifted absorption spectra.
Since the principal axes of .mu. and .beta. are nearly collinear for typical molecular dipoles, it has been proposed that molecular dipoles could be tethered together so as to cause their dipole moments to add constructively. The group of atoms representing the molecular dipoles and the atoms tethering the molecular dipoles together are referred to herein as a "dipole subunit". A dipole subunit is a discrete molecule or a moiety which is a part of a larger molecule, for example, a moiety pendent from a macromolecular backbone.
Previous efforts have succeeded in arranging molecular dipoles in a head-to-tail assembly, and modifying the net dipole moment, however, a major enhancement of .mu..beta. has not been achieved. For example, Katz, H. E., et al, "Head-to-Tail Assemblies of Dipolar, Piperazine-Linked Chromophores: Synthesis, X-ray Structure, and Dielectric Characterization", Journal of the American Chemical Society, (1989), Vol. 111, 7554-7557 teaches head-to-tail dimers and oligomers which have a 6 to 28 percent enhancement in dipole moment in comparison to the monomers.
Katz, H. E., et al, "Chapter 17, Molecular Design for Enhanced Electric Field Orientation of Second-Order Nonlinear Optical Chromophores", Materials for Nonlinear Optics Chemical Perspectives, edit. Marder, S. E. et al, ACS Symposium Series 455, (1991), pp 267-278; discloses an attempt to arrange molecular dipoles in which two chromophores project in parallel directions from a rigid molecular backbone. This reference indicates that the following reaction was performed: ##STR2## and states in relation to that reaction: "A more rigidly parallel pair of bonds for the projection of chromophores are the 1,8 positions of anthracene and anthraquinone. The respective 1,8-dichlorides undergo a limited substitution chemistry, which we extend as shown in {the above equation} to synthesis parallel-directed but weakly dipolar phthalimides. In principle, the use of donor-substituted phthalimide nucleophiles in the reaction of {the above equation} would give a fully additive pair of strong dipoles; however, this has not yet been accomplished." (p 269) The sole reaction product disclosed has the shortcoming that the "weakly dipolar phthalimides" which are linked through one end to the conjugated ring system, would appear by reference to standard tables of Hammett-Sigma values in Lange's Handbook of Chemistry to not even meet the definition of molecular dipoles. Another shortcoming is presented by the linking of the two "dipolar" groups through a conjugated ring system. Dipoles linked by a conjugated system have an electronic absorption spectrum that is red-shifted in comparison to the absorption spectrum of a uncorrelated mixture of the component molecular dipoles.
Levine, B. F., et al, "Second order hyperpolarizability of a polypeptide .alpha.-helix: Poly-.gamma.-benzyl-L-glutamate", The Journal of Chemical Physics, Vol. 65, No. 5, (1 Sep. 1976), pp 1989-1993; teaches a dipole subunit in which molecular dipoles are covalently bonded linearly, but are held in substantially parallel relation by the non-covalent bonding of an alpha-helix. This material has the shortcoming of being highly labile in that the alpha-helix configuration is only present in a narrow range of conditions of solvent, temperature and the like.
Kelderman, E., et al, Angew. Chem. Int. Ed. Engl., Vol. 31, No. 8, (1992) pp 1075-1077; teaches a calix 4!arene having four phenol moieties connected by single methylene bridges. The phenol moieties are functionalized to provide four molecular dipoles. Meredith, G. R. et al, Macromolecules, Vol. 15, (1982), pp 1385-1389; teaches a liquid crystal polymer in which molecular dipoles are correlated by the non-covalent interactions of the liquid crystal.
It is therefore desirable to provide an optical article in which molecular dipoles are not correlated by only non-covalent interactions or only single covalent bonds, which are subject to rotation with heating.